Ultrasonication as an abiotic elicitor - effects on antioxidant capacity and overall quality of romaine lettuce

Fresh produce is important for human health. Various minimal processing technologies have been developed over years to meet the growing demands of consumers for high quality and “fresh-like” fruits and vegetables. However, there have been few food processing researches emphasizing on stimulating the biological system of food itself to achieve an enhanced postharvest quality. It is hypothesized that low acoustic power density (APD) ultrasound, as a form of physical energy, could stimulate the biological stress defense response system of a plant to increase its own secondary metabolite accumulation to achieve added nutritional value and better quality retention. The effect of ultrasound treatment at low APDs on antioxidant capacity and overall quality of Romaine lettuce was evaluated. Whole leaf lettuce was treated with ultrasound (25 kHz) at APD of 26 W/L for 1 - 3 minutes and stored at room temperature for up to 150 hours. Quality indices examined included color, texture, total phenolics, antioxidant capacity, and sensory properties. Phenylalanine ammonia-layse (PAL) activity of lettuce from different treatments was monitored during storage. There were no differences in sample quality attributes between ultrasound treatment and control immediately after treatment. During storage, Romaine lettuce samples treated by ultrasound for 2 min and 3 min exhibited an increase in PAL activity, resulting in production of phenolic compounds as secondary metabolites and enhancement of antioxidant capacity. Ultrasonic application did not cause sample deterioration, and under certain conditions it delayed enzymatic browning and maintained better overall quality. A hypothetical model for the effect of low APD ultrasound as an abiotic elicitor on fresh produce was proposed based on the present study and evidence from reports on responses of cell cultures to ultrasonication. Further studies are needed to achieve a better understanding of the physiological responses of fresh produce to ultrasound treatment, which would help obtain value-added fruit and vegetable products